美國國家航空暨太空總署 (NASA) 過去曾經擁有 5 架自製的載人太空梭 (Space Shuttle)，後來由於太空梭過於老舊，維護成本高、安全性低，在2011年最後一架太空梭 (奮進號) 退役後，就再也沒有自製的太空梭了。
NASA 於 2014 年與波音及 SpaceX 簽約研發可載太空人的新型太空船，希望能重新載送太空人到國際太空站 (International Space Station, ISS)。國際太空站是世界上在軌運行最大的太空站，主要由美國 NASA、俄羅斯航太太空活動國有公司 (Roscosmos)、歐洲太空總署 (ESA)、日本宇宙航空研究開發機構 (JAXA) 和加拿大太空局 (CSA) 共同營運。
NASA 於 2014 年與波音及 SpaceX 簽約研發可載太空人的新型太空船，希望能重新載送太空人到國際太空站 (International Space Station, ISS)。國際太空站是世界上在軌運行最大的太空站，主要由美國 NASA、俄羅斯航太太空活動國有公司 (Roscosmos)、歐洲太空總署 (ESA)、日本宇宙航空研究開發機構 (JAXA) 和加拿大太空局 (CSA) 共同營運。
美國過去五架太空梭的命運
為方便太空人往返於地球和太空，可重複使用的航太運載器「太空梭」得以誕生。美國歷來 5 架服役的太空梭，2 架都全機盡毀，折損率高達 40%，不幸讓 14 名太空人喪命，成為史上最致命的航空器。5 架服役的太空梭整理如下：
- 哥倫比亞號 Columbia (1981 年首飛，2003 年解體)
- 挑戰者號 Challenger (1983 年首飛，1986 年爆炸)
- 發現號 Discovery (1984 年首飛，2011 年退役)
- 亞特蘭蒂斯號 Atlantis (1985 年首飛，2011 年退役)
- 奮進號 Endeavour (1992 年首飛，2011 年退役)
太空梭 1981 至 2011 年服役 30 年間，總計執行 135 次任務，載過近 600 名太空人、和 ISS 對接 33 次、發生 2 次致命意外。在 2011 年最後一架太空梭 (奮進號) 退役後，美國就只能租用俄羅斯的太空船運送太空人往返地面和 ISS。延伸閱讀：《知識》美國NASA - 波音747「太空梭運輸機」
美國太空梭失事率高，可能和本身設計有關
2011年太空梭退役至今，美國都沒有載人太空船可以取代，那為什麼美國當初要放棄太空梭呢？
太空梭是美國阿波羅計畫成功登陸月球之後，研發的一款可重複使用的載人航空器。意思是取代只用一次的傳統火箭，降低發射成本，達到1年發射50次高頻率探索和開發太空。
傳統火箭發射系統，是將載人太空船搭載在火箭頂部，遠離下方燃料缸和底部引擎。一旦火箭發生故障，太空船亦有機會迅速與火箭分離逃脫，救回太空人性命。
然而，美國太空梭的設計，是把太空梭本體 (Orbiter) 直接綁在碩大的外燃料箱 (External Tank, ET) 上面，若燃料箱發生故障，承受風險相對高。除此之外，位於 ET 兩側的 2 枚固體助推火箭 (Solid Rocket Booster, SRB)，因為使用固體燃料，一旦點燃就無法熄火，出故障時難以扭轉局面，相當危險。
1986 年挑戰者號太空船升空後爆炸，就是因為其中一枚 SRB 密封結構失效所致。(註：固體燃料常用於導彈而非載人航空器，太空梭之所以使用 SRB，有人說是為了讓導彈製造商參與項目的政治決定)
太空梭系統雖然做到太空梭本體、固體助推火箭能夠重用，但太空梭發射成本，其實要遠遠高於只用一次的傳統火箭。太空梭本體每次載運太空人返回地球後，由於承受了重返大氣層的高溫高壓，引擎都要拆出來大修，再重新安裝回去。
太空梭成本高、不安全的因素，美國決定放棄這套系統。2011年至今，都是租用俄羅斯的太空船來使用。
太空梭系統雖然做到太空梭本體、固體助推火箭能夠重用，但太空梭發射成本，其實要遠遠高於只用一次的傳統火箭。太空梭本體每次載運太空人返回地球後，由於承受了重返大氣層的高溫高壓，引擎都要拆出來大修，再重新安裝回去。
太空梭成本高、不安全的因素，美國決定放棄這套系統。2011年至今，都是租用俄羅斯的太空船來使用。
美國重返自製太空船，找民間公司一起合作
NASA 於 2014 年，分別與波音及 SpaceX 簽約，研發可載太空人的新型太空船，希望能載送太空人到國際太空站。
比起政府主導的作法，找商業公司合作整體成功機率比較高。商業公司效率通常比政府好，讓商業公司分擔重任，國家給予技術與經費支持，鼓勵其開發往返太空站和地面的太空飛行器，商業載人開發計畫 (Commercial Crew Program) 應運而生。
根據 NASA 的計劃，波音和 SpaceX 公司將獲得研發資金補助，完成可搭載七人的太空船的研製工作。這些研發資金將分階段付給，波音和 SpaceX 都必須滿足研發的具體技術標凖才能獲得資金。
波音研發的太空船名稱為 CST-100 Starliner；SpaceX 研發的太空船名稱為 SpaceX Dragon。兩者目標都是將太空人送到 ISS，結束依賴俄羅斯太空船的狀況。
SpaceX 成功將太空人送往國際太空站
台灣時間 2020 年 5 月 31凌晨 3 點 23 分，SpaceX Dragon 在美國甘迺迪太空中心發射場正式發射成功。約 19 個小時後，載人飛龍號與國際太空站成功對接，美國又開啟了全球商業載人航天新時代。SpaceX 的設計理念都圍繞著「可重複使用」這個概念。SpaceX 已經成功透過可重複火箭-獵鷹9號 (Falcon 9) 將星鏈計畫的衛星發射升空。星鏈計畫使用的是低軌道衛星，特色在於傳輸速率較海底電纜及光纖快。這些衛星將為全球 80% 地區提供高速衛星網路服務。這對收訊不好的偏遠地區居民來說，未來在家上網會更方便，急救人員前往偏遠地區進行遠程醫療服務時也能更即時通訊。如果火箭沒有快速重複利用的特性，基本上星鏈計畫佈署衛星也不會這麼有效率。
火箭回收再利用可降低成本、加速太空飛行發展的目標。SpaceX 火箭是垂直著陸回收的，而不像太空梭助推器那樣透過降落傘落到海上回收。可重複使用的火箭能夠降低發射成本，使得未來殖民火星成為一個切實可行的計畫。
火星計畫
SpaceX 會先派遣貨運太空船到火星，主要任務是發現最佳水源，目前正在研究以水作為火箭動力。其它任務還包括在火星上放置電力、開礦與生命支援基礎設備，以支援未來任務所需。SpaceX公司在上火星之前，也計畫將兩名付費旅客飛往月球，能夠穩定的登月，未來才有機會常態性的往返火星。如果SpaceX真的能夠實現載人往返月球的目標，那將會是自1972年阿波羅17號太空船以來，人類首次再度造訪我們的最近的鄰居 ─ 月球。
波音 CST-100 也成功將太空人送往國際太空站
台灣時間 2024 年 6 月 5 日 晚上 10 時 52 分，波音公司製造的太空船星際飛機 (CST-100 Starliner) ，搭載了兩名 NASA 資深太空人，歷經 27 小時的飛行後，也成功與國際太空站安全對接。這項任務是波音公司首次載人飛行，也是自 2020 年 SpaceX 之後，NASA 再次使用美國製太空船載人前往國際太空站。
-
The Spacecraft Development by NASA, Boeing, and SpaceX
The United States National Aeronautics and Space Administration (NASA) once operated five self-built Space Shuttles. However, due to the aging of the Space Shuttles, high maintenance costs, and low safety, the last Space Shuttle (Endeavour) was retired in 2011, and no new self-built Space Shuttles have been developed.
In 2014, NASA signed contracts with Boeing and SpaceX to develop new manned spacecraft in hopes of sending astronauts back to the International Space Station (ISS). The International Space Station is the largest space station in orbit in the world and is operated jointly by NASA, the Russian State Corporation for Space Activities (Roscosmos), the European Space Agency (ESA), the Japan Aerospace Exploration Agency (JAXA), and the Canadian Space Agency (CSA).
The Fate of the Five Space Shuttles Used by the United States
To facilitate the transportation of astronauts between Earth and space, the reusable space shuttle was born. Of the five space shuttles that have served the United States, two were completely destroyed, resulting in a loss rate of 40% and the tragic deaths of 14 astronauts, making them the deadliest aircraft in history. The five space shuttles that served are as follows:
- Columbia (First flight: 1981, Disintegrated: 2003)
- Challenger (First flight: 1983, Exploded: 1986)
- Discovery (First flight: 1984, Retired: 2011)
- Atlantis (First flight: 1985, Retired: 2011)
- Endeavour (First flight: 1992, Retired: 2011)
Between 1981 and 2011, the space shuttles served for 30 years, completing a total of 135 missions, carrying nearly 600 astronauts, docking with the ISS 33 times, and experiencing two fatal accidents. Since the retirement of the last space shuttle (Endeavour) in 2011, the United States has had to rely on leasing Russian spacecraft to transport astronauts to and from Earth and the ISS.
The High Failure Rate of the US Space Shuttle and Its Potential Design Flaws
Since the retirement of the Space Shuttle in 2011, the United States has lacked a manned spacecraft to replace it. But why did the US abandon the Space Shuttle in the first place?
The Space Shuttle was a reusable manned spacecraft developed after the success of the US Apollo program's Moon landings. It was designed to replace disposable rockets, lower launch costs, and enable frequent space exploration and development, with a target of 50 launches per year.
In traditional rocket launch systems, the manned spacecraft is mounted on top of the rocket, away from the lower fuel tanks and bottom engines. If a rocket malfunction occurs, the spacecraft has a chance to quickly separate from the rocket and escape, saving the lives of the astronauts.
However, the design of the US Space Shuttle placed the orbiter (the main body of the spacecraft) directly on top of the massive external fuel tank (ET). If the fuel tank malfunctioned, the orbiter would be at a much higher risk. Additionally, the two solid rocket boosters (SRBs) located on either side of the ET used solid fuel, which cannot be extinguished once ignited. In case of a malfunction, the situation would be difficult to reverse, making it quite dangerous.
The 1986 Challenger Space Shuttle explosion was caused by the failure of the sealing structure in one of the SRBs. (Note: Solid fuel is commonly used in missiles, not manned spacecraft. Some argue that the use of SRBs in the Space Shuttle was a political decision to involve missile manufacturers in the project.)
While the Space Shuttle system allowed for the reuse of the orbiter and SRBs, the actual launch cost of the Space Shuttle was far higher than that of disposable rockets. After each trip carrying astronauts back to Earth, the Space Shuttle's engines had to be removed and overhauled due to the extreme heat and pressure of re-entry into the atmosphere before being reinstalled.
Due to the high cost and safety concerns of the Space Shuttle, the US decided to abandon the system. Since 2011, it has been relying on Russian spacecraft.
The US Returns to Building Its Own Spacecraft by Collaborating with Private Companies
In 2014, NASA signed contracts with Boeing and SpaceX to develop new manned spacecraft capable of carrying astronauts to the International Space Station (ISS).
Compared to the traditional government-led approach, partnering with commercial companies offers a higher chance of overall success. Commercial companies are generally more efficient than government agencies. By sharing the burden with commercial companies and providing them with technical and financial support, the US government encouraged the development of spacecraft capable of round-trip flights between the ISS and Earth. This led to the creation of the Commercial Crew Program (CCP).
Under NASA's plan, Boeing and SpaceX would receive development funding assistance to complete the construction of their respective seven-person spacecraft. This funding would be provided in phases, and both Boeing and SpaceX would need to meet specific technical milestones to receive the funding.
Boeing's spacecraft is named CST-100 Starliner, while SpaceX's spacecraft is named SpaceX Dragon. Both aim to transport astronauts to the ISS, putting an end to the reliance on Russian spacecraft.
SpaceX Successfully Launches Astronauts to the International Space Station
On May 31, 2020, at 3:23 AM Taiwan time, SpaceX's Dragon spacecraft successfully launched from the Kennedy Space Center in the United States. About 19 hours later, the Crew Dragon successfully docked with the International Space Station, marking a new era of global commercial manned spaceflight for the United States.
SpaceX's design philosophy revolves around the concept of "reusability." SpaceX has already successfully launched satellites for its Starlink project using the reusable Falcon 9 rocket. Starlink uses low-orbit satellites that offer faster transmission speeds than undersea cables and fiber optics. These satellites will provide high-speed satellite internet service to 80% of the world's population. This will make it easier for people in remote areas with poor reception to access the internet at home, and it will also enable more immediate communication for emergency personnel providing remote medical services in remote areas. Without the ability to quickly reuse rockets, the deployment of satellites for the Starlink project would not be as efficient.
Recovering and reusing rockets can lower costs and accelerate the development of spaceflight. SpaceX rockets land vertically, unlike Space Shuttle boosters that parachute into the sea for recovery. Reusable rockets can lower launch costs, making future colonization of Mars a more achievable goal.
Mars Plan
SpaceX will first send cargo spacecraft to Mars, with the primary mission of locating the best water sources. The company is currently researching the use of water as rocket fuel. Other tasks include setting up power, mining, and life support infrastructure on Mars to support future missions. Before landing on Mars, SpaceX also plans to fly two paying passengers to the moon. Stable lunar landings will pave the way for regular round trips to Mars in the future. If SpaceX can achieve the goal of manned round trips to the moon, it would be the first time humans have returned to our nearest neighbor since the Apollo 17 mission in 1972.
